What are 50 Ohm terminals used for?

[thilo]

Hi,

what are these 50 Ohm terminals used for that Dave uses in his latest blog? And maybe you could explain to me what this 50 Ohms internal resistance is that Dave talks about?



Thanks,

Thilo.

[alm]

Try searching for transmission line theory in any proper EE textbook or online.

[Mechatrommer]

what are these 50 Ohm terminals used for that Dave uses in his latest blog? And maybe you could explain to me what this 50 Ohms internal resistance is that Dave talks about?

if you have signal source with its output impedance of 50ohm, you need to connect it to another 50ohm internal input impedance device to get maximum power transfer. my 2cnt i just learnt.

[ejeffrey]

A 50 ohm terminator is just a little bnc cap that has a resistor (you guessed it, 50 ohms) between the signal and the shield / ground.

Basically, you need the terminating resistor whenever your cable is a 'transmission line' -- that is, the propagation time of your signal cannot be considered instantaneous.  1 meter of coax has a propagation delay of 5 nanoseconds and a round-trip time of 10 nanoseconds.  If your signal changes fast enough that you cannot always consider the input and output to be at the same voltage you will have a problem due to signal reflections.  The exact cutoff where you need proper termination depends a bit on your application, the driving circuit, and whether there are any intermediate 'taps', but basically even a relatively small fraction of a wavelength can cause problems.

[kevyk]

what are these 50 Ohm terminals used for that Dave uses in his latest blog? And maybe you could explain to me what this 50 Ohms internal resistance is that Dave talks about?
[/quote]

These are line terminators that are sometimes used particularly in high frequency applications to terminate the open dangling end of a test cable. This reduces spurious unwanted external (or internal) interference from migrating into your testing set up and messing with your measurements. He was using them just to short the scope inputs together through the built in 50 ohm resistor.

[w2aew]

<...snip...>
 And maybe you could explain to me what this 50 Ohms internal resistance is that Dave talks about?

The internal resistance that Dave is talking about is the "output impedance" of the signal generator.  The simplest way to think about this is to think of the output of the generator being composed of an ideal generator voltage source followed by a series 50 ohm resistor.  This is done on generators that are able to produce high-speed signals (for the transmission line issues/reasons touched on above).  By providing the 50ohm output impedance, the generator will be well matched to 50ohm transmission lines, and will help to squash any reflections coming back down the line from the load.

The reason it is important to think about in the example that Dave used is that he used the output impedance of the generator as part of a voltage divider circuit when he connected it to 12.5ohm load.  By knowing the output impedance of the generator, and the load impedance, you can calculate the attenuation the signal would experience at the load.

Many high speed function generators have 50ohm output impedances.  You have to take this into account when setting the output voltage level, since the voltage at the load may be affected by the load impedance.  Many generators give you the ability to specify the load impedance so that the output voltage level will be correct as displayed.  Some generators don't help you out this way.  A lot of people get easily confused when the output voltage of their generator is twice as big as they expected (because they connected it to a high impedance load, when the generator was expecting a 50ohm load).